Far-right supporters holding aloft American flags attempted to perform a “citizen’s arrest” of London mayor and Trump critic Sadiq Khan on Saturday.

Khan was delivering a speech at the left-wing Fabian Society when protesters from a group called the White Pendragons, shouting pro-Trump and pro-Brexit slogans, disrupted his talk, according to multiple reports.

Khan on Friday had said that President Donald Trump had “got the message” that he was “not welcome in London” after the president canceled a planned state visit.

In a video posted on Twitter, one protester says "Ladies and gentlemen, we're here today to make a non-violent, peaceful citizen's arrest."

As security appeared to usher out the protesters, they were jeered and heckled by the crowd who had come to see Khan’s speech.

But one protester refused to leave.

"We stand under common law jurisdiction,” he said, as quoted by Sky News.

"If you touch us, you'll be done for common assault. Please stand back, do not touch us.

"We're not leaving, we've paid for a ticket."

Khan reportedly sat down and flicked through a newspaper as he waited for the commotion to die down and the protesters to be escorted out.

After they had been led out, Khan aimed another jab at President Trump.

“It is a pleasure to be here even though we were distracted by the actions of what some would call very stable geniuses,” he said, referring to last week's Trump tweet in which he referred to himself as a "very stable genius."

A longstanding Trump critic, Khan was attacked by the president for urging calm after the London Bridge terror attack last June, in which eight people were killed.

“At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!" the president tweeted.

After leading a vigil for the victims of the attack, Khan told the BBC “We aren’t going to allow anybody, whether it’s Donald Trump or anybody else, to divide our community.”